A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Immersion techniques have been introduced into lithographic systems to enable improved resolution of smaller features. In an immersion lithographic apparatus, a liquid layer of a liquid having a relatively high refractive index is interposed in a space between a projection system of the apparatus (through which the patterned beam is projected towards the substrate) and the substrate. The liquid covers at last the part of the wafer under the final lens element of the projection system. Thus, at least the portion of the substrate undergoing exposure is immersed in the liquid. The effect of the immersion liquid is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid than gas. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.)
In commercial immersion lithography, the liquid is water. Typically the water is distilled water of high purity, such as Ultra-Pure Water (UPW) which is commonly used in semiconductor fabrication plants. In an immersion system, the UPW is often purified and it may undergo additional treatment steps before supply to the immersion space as immersion liquid. Other liquids with a high refractive index can be used besides water can be used as the immersion liquid, for example: a hydrocarbon, such as a fluorohydrocarbon; and/or an aqueous solution. Further, other fluids besides liquid have been envisaged for use in immersion lithography.
In this specification, reference will be made in the description to localized immersion in which the immersion liquid is confined, in use, to the space between the final lens element and a surface facing the final element. The facing surface is a surface of substrate or a surface of the supporting stage (or substrate table) that is co-planar with the substrate surface. (Please note that reference in the following text to surface of the substrate W also refers in addition or in the alternative to a surface of the substrate table, unless expressly stated otherwise; and vice versa). A fluid handling structure present between the projection system and the stage is used to confine the immersion to the immersion space. The space filled by liquid is smaller in plan than the top surface of the substrate and the space remains substantially stationary relative to the projection system while the substrate and substrate stage move underneath. Other immersion systems have been envisaged such as an unconfined immersion system (a so-called ‘All Wet’ immersion system) and a bath immersion system.
An alternative to immersion lithography is EUV lithography, in which the radiation beam is formed of EUV radiation, e.g. having a wavelength in the range of from 5 nm to 20 nm. EUV radiation can be generated by a plasma source or a free-electron laser, for example. In EUV lithography, the beam path, including the mask and substrate, are kept in a near-vacuum and reflective optical elements are mostly used. This is because EUV radiation is strongly absorbed by most materials. A low pressure of hydrogen gas may be present, e.g. to assist in cleaning contaminants when a plasma source is used.
When a substrate is exposed in a lithographic apparatus, energy from the projection beam is absorbed by the substrate and therefore the substrate heats up. The heating is local to the target portion being exposed and therefore any thermal expansion of the substrate due to the heating can lead to distortion of the substrate. Substrate distortion can lead to overlay errors, for example between successive layers, or even exposures of the same area when multiple patterning techniques (using multiple exposures) are used to expose a single layer. The heating that may occur depends on a number of factors. These factors may include (in a non-limited list) the duration of an exposures such as for a whole substrate or of a single scan, details of the photo-sensitive layer and, in an immersion-type lithographic apparatus, the flow rate of immersion liquid during the exposure. In an EUV lithographic apparatus the problem of local heating can be particularly acute because the near-vacuum environment conducts less heat from the substrate than does the immersion liquid in an immersion-type lithographic apparatus. Flows of such gases as are present in an EUV lithographic apparatus do however affect the conduction of heat from the substrate. It is therefore difficult to predict the nature and magnitude of any distortion of a substrate that may occur when it is exposed. The distortion has therefore been determined empirically, by measuring overlay errors in test exposures. However, with such an approach it is difficult to separate overlay errors caused by thermal distortion of the substrate from overlay errors deriving from other causes.
A thermal test substrate (or wafer) supplied by KLA-Tencor of Milpitas, Calif. is known. This thermal test substrate includes about 10 to 100 negative temperature coefficient temperature sensors. The temperature sensors are fixed at various points in a 300 mm silicon wafer. The temperature sensors are connected to readout electronics located centrally in the substrate. The thermal test substrate has the approximate dimensions of a production wafer. The thermal test substrate is processed through a lithographic apparatus as if it were a production substrate. The thermal test substrate records the temperature recorded by the negative temperature coefficient temperature sensors periodically. However, this thermal test substrate does not provide enough accurate information to satisfy increasingly strict limits on overlay errors imposed by the desire to image patterns of smaller critical dimension.